Fluoropolymer finish compositions are used to form non-stick surfaces on a wide variety, of substrates, such as metals, e.g. aluminum, cold-rolled steel, stainless steel, and ceramics, such as glass, enamel, and pyroceram, for a wide variety of products, e.g. cookware, bakeware, iron soleplates, waffle irons, ice-cube trays, snow shovels and plows, chutes and conveyors, saws, hoppers and other industrial containers. The finish formed on these materials for these as well as other products may be a single layer of fused composition or multiple layers of composition, in which the layer in direct contact with the substrate is called the primer layer for the overcoat fluoropolymer-containing layer(s). In all of these applications it is necessary for the layer in contact with the substrate to adhere to it, so that the non-stick finish provides this attribute in the particular service of the coated product.
Fluoropolymer finish compositions have preferably been prepared in the form of an aqueous dispersion of the fluoropolymer, and the aqueous dispersion has then been used to coat the substrate, followed by drying and baking to fuse the coating to form the layer adhered to the substrate. In addition to fluoropolymer, the dispersion has contained polymer binder in solution in the dispersion medium and pigment dispersed in the dispersion. In some cases, the dispersion also has inorganic film hardener dispersed therein, which provided durability, e.g. scratch resistance, to the non-stick coating.
It has been the practice in making these compositions to (a) obtain the fluoropolymer aqueous dispersion from the fluoropolymer manufacturer, (b) form the solution of polymer binder in water, which may also include organic solvent for the binder, which solvent is miscible with water, and (c) mill pigment in water which contains a surfactant, to form a mill base, and then blending the dispersion of (a), the solution of (b), and the mill base of (c) together to form the coating composition. This was the blending by techniques normal in the art referredto in U.S. Pat. No. 5,168,013 (Tannenbaum). When filler film hardener was used, this was simply added to the dispersion (a). U.S. Pat. No. 4,139,576 (Yoshimura et. al.) in Examples 1 and 5 discloses the separate milling of polymer binder and pigment and then blending these milled products with polytetrafluoroethylene. U.S. Pat. No. 4,087,394 (Concannon) discloses the mixing together of an aqueous dispersion of TFE/HFP copolymer, a black mill base, and an aqueous solution of binder (polyamide acid salt, also referred to as polyamic acid salt) in Example 1.
There remains file need to improve the durability of the fluoropolymer finish to its; substrate, especially with regard to adhesion of the finish to the substrate.